Eddy current induction

Keywords coin validation, eddy current, inductive sensors, conductivity, coin validation, electromagnetic sensors... 

The journal center locus is measured by four gap sensors of eddy-current-inductive type. They are fixed in two perpendicular directions at both ends of the test bearing. 

The principle physical phenomenon of applying the eddy current method for evaluating the amount of residual austenite in the structure of quenched steel is magnetic induction, involving the influence of the changeable magnetic field on the studied area, found under the probe. 

In effect of such activity eddy currents in the studied area are induced, producing own magnetic fields which following Lenz s rule are directed adversely to the induction field this decreasing its intensity (Fig. 1). 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

First, the eddy current density is damped while penetrating into the conductor (penetration effect). Here the frequency dependence of the penetration depth implies that for deep lying cracks low frequencies must be used for obtaining a sufficient current density in the vicinity of the crack. Secondly, due to the induction law the induced current density at the surface jco is diminished when using lower frequencies. Therefore, in total, there is a certain excitation frequency which results in a maximum response field from the crack. 

Exciting developments based on electromagnetic induction raced along from that time, giving us the sophisticated products our everyday lives depend on. During most of the period productive uses for eddy current technology were few and few people believed in it as a usefiil tool eddy currents caused power loss in electrical circuits and, due to the skin effect, currents flowed only in the outer surfaces of conductors when the user had paid for all the copper in the cable. The speedometer and the familiar household power meter are examples of everyday uses that we may tend to forget about. The brakes on some models of exercise bicycle are based on the same principle. 

Induction laws and experiments show that the impedance of a coil crossed by an AC current put near a conductive piece is modified by the creation of eddy currents. The presence of an anomaly in this material structure modifies the impedance of the generating coil. The impedance variation measure is at the root of non destructive testing by eddy currents. Any variation inside a piece (variation of conductivity or permeability) modifies the intensity and the course of the eddy currents and consequently the coil impedance. 

Eddy currents and the magnetic flux that is associated to them are proportional to the radial distance of the coil center. The magnetic flux is proportional to the probe induction and consequently to the passing current. The theoretic calculation of this induction is given by the following equation ... 

With the presence of material, the inductance variation follows an exponential law. The increase of the coil resistance is more significant in function of the frequency, due to the generation of eddy currents in the material. 

Several types of Eddy current probes were used with the SQUID system and the commercial system as well. High inductance wire wound probes with a ferritie eore and low induetance planar thick frhn coils were applied. The wire wound probe is the commonly used probe for high resolution conventional testing. The low inductance planar cod is more suited to be apphed in combination with the SQUID system. It is well adapted for surfaee defects and shallow defects. 

As stated above the SQUID amplifier demands a low inductance Eddy current probe in order to be able to amplify signals up to 1 MHz. Low inductance Eddy current probes can be obtained by reducing the number of turns and by loosing the magnetic coupling between the tums. So magnetic cores should be avoided as well as tight wounded tums. For this purpose planar coils are the best... 

L inductance of the solenoid in presence of eddy- current If we define the phases relative to the current then... 

A common known method to get eddy-current informations about material flaws is the measurement of real- and imaginary part of the complex impedance of a coil in absolute circuit. The measurement, shown in this paper, are done with an impedance analyzer (HP4192A). The device measures the serial inductance L, and the serial resistance Rs of the complex impedance with an auto-balance bridge measurement circuit [5]. 

An induction furnace, where the heating is due to eddy current losses itiduced by the magnetic field. 

The enclosure is constructed of non-magnetic material, generally aluminium, in view of its low cost and weight as compared to copper The nonmagnetic material eliminates hysteresis and eddy current losses in the enclosure, as i result of mutual induction,... 

---